Field of the Invention
The present invention relates to an imaging lens device, and more particularly, to an imaging lens device that optically captures an image of a subject by an imaging lens system and outputs the captured image as an electronic signal by an image sensing device, said imaging lens device being a principal component of a camera such as a digital camera, a video camera, or a camera incorporated in or externally attached to an apparatus such as a digital video unit, a personal computer, a mobile computer, a portable telephone, or a personal digital assistant (PDA).
Description of the Prior Art
Solid-state image sensing devices such as charge coupled devices (CCDs) disposed on image forming surfaces of various kinds of video cameras and digital cameras have been reduced in size year by year with advances in technology, and this requires imaging lens systems to be also reduced in size. As fixed-focal-length imaging lens systems compatible with small-format solid state image sensing devices, the following are known: ones comprising three lens elements (Japanese Laid-open Patent Application No. H10-319315, etc.); ones comprising two lens elements (Japanese Laid-open Patent Application No. H7-333494, etc.); and ones comprising one lens element (Japanese Laid-open Patent Application No. H6-88939, etc.).
In the three-element imaging lens system described in Japanese Laid-open Patent Application No. H10-319315, the cost is high since the number of lens elements is large, and the overall size of the lens system is large. In the two-element imaging lens system described in Japanese Laid-open Patent Application No. H7-333494, since the lens system comprises two plastic lens elements, variation in back focal length due to increase in temperature is too large, so that performance cannot be maintained. In the one-element imaging lens system described in Japanese Laid-open Patent Application No. H6-88939, since image plane quality and distortion are poor, high image quality cannot be maintained. Thus, these conventional examples are not optimum because they do not achieve size reduction and performance maintenance in good balance.